1. Field of the Invention
The present invention relates to a magnetoresistive effect device and a magnetic random access memory using the same, for example, a magnetoresistive effect device using a spin-injection writing scheme.
2. Description of the Related Art
In recent years, there have been proposed a large number of solid-state memories on which information is recorded on the basis of new principles. Of the solid-state memories, as a solid-state magnetic memory, a magnetoresistive random access memory (MRAM) using a tunneling magnetoresistance (TMR) is known. The MRAM uses a magnetoresistive (MR) device which exerts a magnetoresistive effect as a memory cell to store information in a memory cell depending on a state of magnetization of the MR element.
An MR element includes a layer having variable magnetization and a layer having fixed magnetization. When the direction of magnetization of the variable-magnetization layer is parallel to that of the fixed-magnetization layer, a low resistance state appears. When the directions are opposite to each other, a high resistance state appears. The difference between the resistance states is used to store information.
As a method of writing information in the MR element, a so-called current-magnetic-field writing scheme is used. In this scheme, interconnects are arranged near the MR element, and a magnetic field generated by a current flowing in the interconnects changes states of magnetization of the MR element. When the MR element is reduced in size to shrink the MRAM, the MR element increases in coercivity Hc. For this reason, in the MRAM using the magnetic-field writing scheme, with advance of shrinkage, a current required for writing tends to increase. As a result, shrinkage of a cell size for a large capacity over 256 Mbits and a low-current configuration is not easily compatible.
A wiring scheme using spin momentum transfer (SMT) as a writing scheme (spin-injection writing) that overcomes the above problem is proposed (specification of U.S. Pat. No. 6,256,223). In the spin-injection writing scheme, a current is conducted perpendicularly to surfaces where respective films face in a device (MR element) which achieves tunneling magnetoresistance to change (switch) states of magnetization of the MR element.
In magnetization switching by spin injection, a current Ic required for magnetization switching is accurately regulated by a current density Jc. Therefore, when an area of surface through which a current of the MR element flows decreases, the injection current Ic to switch magnetization also decreases. With constant current density for writing, reduced-sized MR element requires decreased current Ic. This makes the spin-injection writing scheme outperform the magnetic-field writing scheme in scalability in principle.
However, when the spin-injection writing scheme is used to realize an MRAM, a current required for magnetization switching is larger than that generated by a select transistor, which is frequently used to realize an MRAM. For this reason, the MRAM cannot be substantially operated as a memory.